In the recent past, substantial research and development resources have been directed toward small scale Liquid Crystal Display (LCD) and light valve technologies. These miniature LCD assemblies are typically employed in high resolution projection displays, such as a reflective LCD projectors, SXGA formats (1,280×1,024 pixel resolution) and even HDTV formats (above 1,000 line resolution), or the like.
Briefly, as shown in FIGS. 1 and 2, a conventional small scale LCD assembly 20 is illustrated including a die 21 having a pixel array 22. This pixel array 22 is typically composed of rows and columns of electrically conductive pathways each forming an individual pixel (not shown). Each pixel can be individually changed to an “on” condition by selecting the appropriate row and column of pixel array 22. Positioned around or concentrated on one end of the pixel array are a plurality of die bond pads 23 which are internally connected to the pixel array 22 to enable operational control thereof. Selection of the appropriate pixel is controlled by control circuitry, either included within the die 21 or external to the die 21. In either configuration, external control signals may be used to control the functions of the die 21.
As best viewed in FIGS. 2 and 3, a transparent glass plate 24 is typically placed over the die 21 and the pixel array 22, such that a portion of the glass plate 24 overhangs the die 21. The glass plate 24 is usually affixed to die 21 through an adhesive seal 25 which together cooperate to define a sealed volume encompassing the pixel array 22. This sealed volume is then commonly filled with a solution 26 of liquid crystal material such as Twisted Nematic Liquid Crystals (TNLC). To facilitate grounding of the glass plate 24, a conductive coating (not shown) may be deposited over the undersurface 28 thereof.
The die 21 is typically rigidly or semi-rigidly mounted to a substrate 27 for mounting support and to facilitate heat conductive dissipation for the die. A conductive adhesive 29 (FIG. 3), such as a conductive epoxy, is generally applied to the undersurface 28 of the die 21 to adhere the die directly to the top surface of the substrate 27. In this manner, a heat conductive pathway is created directly between the die and the substrate to dissipate heat generated by the die.
The substrate 27 generally includes a plurality of substrate bond pads 30 which are typically wire bonded to the die bond pads 23 through bonding wires 31. Finally, an encapsulating material 32 is applied to seal die 21 to substrate 27. The encapsulating material 32 (FIG. 3) normally encapsulates the bonding wires 31 and the internal elements of die 21 without obscuring a view of the pixel array 22 through the glass plate 24.
By activating the appropriate pixels, the corresponding liquid crystals in the TNLC, deposited in sealed volume, are caused to either align or rotate through an appropriate polarizer. Upon alignment, light is permitted to pass through the aligned crystals and the adjacent glass plate, thus appearing light in color. In contrast, when the liquid crystals are rotated, light is prevented from passing therethrough and, hence the glass plate 24, so that the corresponding pixel appears dark in color.
One important aspect in the proper operation of these small scale LCD or light valve assemblies is the maintenance of proper distance uniformity (typically about 2–4 μm) between the pixel array and the undersurface 33 of the glass plate. Variances in these distances may often times cause the pixel array to function improperly or cause operational failure.
One problem with conventional rigid display device constructions where the substrate 27, the glass plate 24 and the silicon die 21 are all attached are optical defects due to warping. Since the structures are composed of different materials or composites that have different coefficients of expansion, they expand at different rates and cause each other to warp. As a result of this deformation, depending in part upon the construction processes, significant residual stresses may be induced upon the cell. At a minimum, these internal stresses cause optical defects such as variations in color uniformity and fringes, and variations in the cell gap thickness which may cause optical shadows. As these optical defects may be produced by deformations as small as 0.25 microns, minor stresses may substantially reduce optical quality.
This is especially true since the undersurface 28 of the die 21 is typically rigidly affixed or attached directly to the substrate 27. For example, when the substrate 27 and the die 21 are both composed of a silicon material, upon heating, the glass plate 24 expansion tends to negatively bow or warp (FIG. 4) at a rate greater than that of the substrate 27. Upon more extensive high temperature thermal cycling during operation, additional occurrence of optical fringes and optical non-uniformity may even further compromise the performance of the LCD assembly.
In contrast, when the die 21 is composed of a silicon material and the substrate 27 is composed of a more conductive material, such as aluminum, upon heating, the substrate expansion tends to positively bow or warp (FIG. 5) the substrate at a rate greater than that of the die 21 and glass plate 24. As viewed in the cross-sectional view of FIG. 5, central thinning of the cell is caused which results in defects such as discoloration and the appearance of optical shadows.
Another cause of optical defects due to stress occurs during construction of the small scale LCD assembly 20. Commonly, the encapsulating material 32 used to protect the bonding wires 31 may surround the glass plate 24 and the silicon die 21. As the encapsulating material 32 is cured, differences in thermal expansion between the encapsulating material 32 and the glass plate 24 or the silicon die 21 may lead to peripheral deformation of the glass plate 24 or the silicon die 21, leading to further stressing and optical defects.
In view of the foregoing, it should be apparent that improved LCD assembly and construction techniques would be desirable.